When molten glass is drawn into sheet form, the glass is stretched or attenuated from an initial delivered thickness to a final sheet thickness. In the overflow downdraw process, where molten glass flows downwardly along opposed converging sides of a forming member and is withdrawn as a single ribbon of glass from the root or bottom edge thereof, the initial thickness of the glass ribbon is measured close to the bottom edge of the forming member, which represents the draw line in such an operation. Single sheets of glass are then separated from the free end of the drawn ribbon.
Obtaining thickness uniformity of the ribbon has been a problem in both updraw and downdraw processes where the thickness characteristics of the final sheet are determined during the attenuation process by both the uniformity of initial thickness and by the uniformity of the glass viscosity. That is, a given thickness variation in the final sheet may be the result of inaccurate metering, imperfections in the glass-contacting sides of the forming member, or by imbalances in the temperature environment of the glass that cause imperfections in the viscosity profile of the glass flowing toward the draw line.
Thickness variation in sheet glass is a problem that has been considered by the industry to be inherent in sheet drawing processes, and may manifest itself in several general types of defects, such as wedge, long period wave variations, and short period wave variations. Wedge is a gross thickness variation in which the ribbon or sheet is thicker at one edge than the other. Long wave variations are those that have considerable amplitude and extent, such as in excess of several inches, and can be measured by gauging the ribbon along a path in a direction transverse to the direction of the draw. Short wave variations are of small amplitude and pitch, such as about three inches or less, and are generally superimposed on the long wave variations.
It has been found that to make distortion-free sheet glass, it is necessary to minimize or compensate for local temperature variations or fluctuations within and around the glass in the zone of ribbon formation. Such local variations in temperature in the vicinity of the draw line cause waves, or alternate thick and thin portions running longitudinally in the vertically drawn ribbon. The longitudinal waves or thickness variations, in turn, cause distortion that is highly objectionable from an optical standpoint, particularly when objects are viewed through the glass at a sharp angle to the waves.
Prior art methods of controlling these thickness variations included flowing air against the molten glass from cooling tubes arrayed along the length of the forming body. The straight cooling tubes were arranged at equal intervals along the forming body length, and positioned so the central longitudinal axis of each tube was perpendicular to a vertical plane passing through the root. Moreover, the cooling tubes were shrouded by an outer tubular shield. Thus, the tubes were rigidly positioned in relationship to the forming body and the glass flow.
Unfortunately, thickness defects in the glass ribbon may not be positionally stable over long periods of time, nor may the lateral position of the ribbon itself be constant. Thus, the pre-positioned and immovable cooling tubes may at a first instance be properly positioned, but at a second time be ill-positioned to effectively control thickness due to movement of the defect or ribbon.
The present invention is directed to an improved method of cooling the flowing molten glass to eliminate, or substantial reduce, the general type of thickness variation identified as short wave variation having a width of several inches or less, and an apparatus therefor.